1. Field of the Invention
The present invention relates to an apparatus for driving a lamp of a liquid crystal display, and more particularly to an apparatus for driving a lamp of a liquid crystal display capable of simplifying a structure of the liquid crystal display and preventing a leakage current from the apparatus for driving lamp.
2. Description of the Related Art
In general, the scope of application of liquid crystal displays has widened due to the lightweight, thinness, and low power consumption of liquid crystal displays. According to this trend, liquid crystal displays are widely used in an office automation machines and an audio/video machines. The intensity of light beam is adjusted in accordance with a video signal applied to a plurality of control switches arranged in a matrix in order to display a desired picture on a screen.
An LCD needs a light source such as a back light. A cathode fluorescent tube (CCFL) may be used as a light source employed as the back light.
The CCFL is a light source tube using a cold emission phenomenon (the electron emission occurring because a strong electric field is applied to a surface of a cathode.) and is frequently used due to low heat generation, high brightness, long life, and full color reproduction. A CCFL like this has a light guide system, a direct illumination system and a reflection system. So a light source tube is adopted in accordance with a requirement of the LCD. The CCFL may have an inverter circuit for obtaining a high power source from a low power source.
Referring to FIGS. 1 and 2, a lamp driving apparatus of an LCD according to the prior art comprises a lamp housing 10 having a plurality of lamps, an inverter block 20 having a plurality of inverters for supplying a lamp driving voltage to each of the lamps, a first integrated circuit substrate 12 having the inverter block mounted thereon, a current detector 30 having a plurality of current detectors for detecting a tube current in each of the inverters, a second integrated circuit substrate 32 having the current detector 30 mounted thereon and a feedback line 36 connected between the current detector 30 and the inverter block 20 for supplying the inverter block 20 with a feedback signal from the current detector 30.
The lamp housing 10 is provided with a mounting space for mounting a plurality of lamps and is stacked on a main support.
Each of the lamps receives the lamp driving voltage from the inverter block 20 to radiate visible light to a liquid crystal panel (not shown).
The first integrated circuit substrate 12 is located on a lateral portion of the main support 2 and is folded toward a rear surface of the main support 2.
The second integrated circuit substrate 32 is located on another lateral portion of the main support 2 and is folded toward a rear surface of the main support 2. A protecting chassis protects the second integrated circuit substrate 32 and is mounted between the second integrated circuit substrate 32 and the main support 2.
The feedback line 36 connects the first and the second integrated circuit board 12 and 32 that are folded onto the rear surface of the main support. The feedback line 36 may have a plurality of signal wires.
As shown in FIG. 3, each of the inverters in the inverter block 20, comprises a switch circuit 24 for switching a voltage from a voltage source (Vin) in response to a switching control signal, a transformer 22 for converting a voltage supplied by switching of the switch circuit 24 to the lamp driving voltage, a pulse width modulation circuit for controlling the switch circuit 24 in response to the feedback signal (FB) from the current detector 30.
The switch circuit 24 comprises at least one switch device switching a voltage from the voltage source (Vin) to the transformer 22 in response to the switching control signal from the pulse width modulation circuit 26.
The transformer 22 has a primary winding connected to the switch circuit 24 and a secondary winding connected to the lamp 40. The both ends of the primary winding are connected to the switch circuit 24 and one end of the secondary winding is connected to a first electrode terminal of the lamp 40 while the other end is connected to a ground voltage (GND). The transformer 22 converts a voltage supplied to the primary winding by a winding ratio of the first and the secondary winding and induces a voltage into the secondary winding. The voltage induced into the secondary winding is supplied to the lamp 40 through the first electrode terminal of the lamp 40 to turn on/off the lamp 40.
The pulse width modulation circuit 26 controls a switching time period of the switch circuit 24 in response to the feedback signal (FB) from the current detector 30. That is, the pulse width modulation circuit 26 controls the voltage to be supplied to the transformer 22 by controlling the switching time period of the switching circuit 24 in response to the feedback signal (FB).
As shown in FIG. 3, each of the current detectors 31 in the current detector 30, as shown in FIG. 3, is connected between the second electrode terminal of the lamp 40 and the ground voltage source (GND) and supplies the feedback signal (FB) corresponding to a tube current value detected from the lamp 40 to the pulse width modulation circuit 26. To this end, each of the current detectors 30 comprises a first resistor (R1) connected between the second electrode terminal of the lamp 40 and the ground voltage source (GND), a variable resistor (RB) connected between the first resistor (R1) and the ground voltage source (GND), a first diode (D1) connected between the pulse width modulation circuit 26 and the a first node (N1) between the second electrode terminal of the lamp 40 and the first resistor (R1), and a second diode (D2) connected between the ground voltage source (GND) and a second node (N2) between the first node (N1) and the first diode (D1).
The first resistor and variable resistor (R1 and RB) detect a current value of the second electrode terminal of the lamp 40 by a divided resistance and result in a detected signal occur on the first node (N1). The feedback signal (FB) which is the detected signal on the first node (N1) is supplied to the pulse width modulation circuit 26 through the first diode (D1). The second diode (D2) cuts off an impulse of a negative potential and maintains a lowest voltage of the feedback signal (FB) to zero (0) voltage.
In the lamp driving apparatus for an LCD according to the related art, a voltage from the voltage source (Vin) is supplied to the primary winding of the transformer 22 by the switching control of the pulse width modulation circuit 26 of the inverter 20. The voltage supplied to the primary winding of the transformer 22 is converted by the first and the secondary winding ratio of the transformer 22 and is induced into the secondary winding. The current induced at the secondary winding of the transformer 22 is supplied to the lamp and thereby the lamp turns on/off. If the lamp 40 turns on/off, the current detector 30 detects the tube current of the lamp and supplies the feedback signal (FB) corresponding to the detection signal detected to the pulse width modulation circuit 26. Accordingly, the pulse width modulation circuit 26 converts the switching time period of the switch circuit 24 in response to the feedback signal (FB) and controls the voltage supplied to the primary winding of the transformer 22.
As shown in FIG. 4, in the lamp driving apparatus of the LCD according to a related, the lamp driving voltage supplied to a plurality of lamps has the same phase. Accordingly, because the leakage current is large, the power consumption becomes large. In detail, when the phase of the driving current supplied to a plurality of lamps is identical and an impedance of each of the lamps is increased, the leakage current becomes large. The impedance is increased by coupling the current/phase of the adjacent lamps and thereby the leakage current becomes large. Accordingly, the driving of the lamp becomes unstable due to the leakage current of each of the lamps.
In the lamp driving apparatus of an LCD according to the related art, because the current detector 30 is connected to the second electrode terminal of the lamp 40, the feedback line making the current detector 30 and the inverter block 20 electrically connected becomes necessary. As a result, there is disadvantage that the structure of the liquid crystal display becomes complicated.